Distinguish international and domestic USPS calculators because service codes returned from usps api are not mutually exclusive

Update USPS shipping rate names and fix a bug in checking max weight for countries.

Add USPS Standard Post and USPS Global Express Guaranteed (GXG)

Fix broken rspec

Fixes #196

Conflicts:
	spree_active_shipping.gemspec

Conflicts:
	spree_active_shipping.gemspec